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Chimps Go Ape Over Older Females

In the 1971 cult film Harold and Maude, a young man finds romance with a septuagenarian. Real life is usually different: Cross-cultural research suggests that men generally prefer to hook up with younger women. The same does not appear to hold true for chimpanzees. A new study finds that the males of that species go for older females in a big way.

Chimp researchers have long suspected that the animals prefer older females as mates. For example, Jane Goodall, who spent nearly half a century studying the chimps at Gombe, Tanzania, noted that some males were extremely attracted to older females. And a 1970s study with two females showed that most males preferred the older one. Yet few rigorous studies with larger numbers of animals have been conducted.

To tackle the question systematically, a team led by anthropologist Martin Muller of Boston University in Massachusetts analyzed the behavior of nearly 3 dozen chimps at Kibale National Park in Uganda between 1996 and 2003. Muller and colleagues looked at four measures of female attractiveness to the opposite sex: the number of times a male approached a female to copulate; the number of males that grouped around an ovulating female; the rate at which high-ranking males copulated with particular females; and the extent to which males appeared to be fighting over females.

For each measure, there was a strong positive correlation between the age of females and how attractive they were to males. And when the researchers combined all of the data to provide a "composite attractiveness index," they found that male chimps were twice as attracted to 45-year-old females as they were to 15-year-old females. That means the chimps are far less fond of females that have just begun to reproduce than of females pushing the end of their golden years. (Female chimps remain reproductively active for most of their lives.)

So why are male chimps and male humans so different in this regard? The researchers suggest that chimps, who are notoriously promiscuous, may value immediate reproductive success over long term fertility. Thus, they're more likely to mate with mature females who have proven their ability to bear offspring. Humans, on the other hand, tend to form long-term mating bonds and thus may opt for younger mates who will produce more offspring over their lifetimes, the team reports in the 21 November issue of Current Biology.

It's a "compelling" argument based on "quite persuasive" data, says Joan Silk, an anthropologist at the University of California, Los Angeles. But primatologist Frans de Waal of Emory University in Atlanta, Georgia cautions that the cross-cultural data on human mating behavior is not solid enough to make definitive comparisons between humans and chimps. "Everyone thinks that men go for younger women," de Waal says, "but all human results are based on questionnaires and on men selecting pictures" of women they find attractive. "We don't know if in real life they do the same."